1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy hydrocarbon oils. More particularly, this invention relates to an improved process for dilution chilling dewaxing waxy hydrocarbon oil stocks in a staged chilling zone wherein cold dewaxing solvent is injected into said zone at a plurality of stages therealong and wherein the cold dewaxing solvent and the waxy oil are substantially instantaneously mixed in each stage as the waxy oil-solvent mixture passes from stage to stage, the improvement comprising adjusting the temperature profile along said zone such that when the waxy feed stock is a 600N waxy oil having a final boiling point of &gt;600.degree. C., preferably &gt;605.degree. C., most preferably &gt;610.degree. C. and a mean boiling point of about 490.degree.-510.degree. C., preferably about 500.degree. C. there is an equal chilling rate/stage profile as compared to an equal temperature drop profile in the staged chilling zone and when the waxy feed stock is a 600N waxy oil having a final boiling point &lt;600.degree. C., preferably &lt;595.degree. C., most preferably &lt;590.degree. C. and a mean boiling point of about 490.degree.-510.degree. C., preferably about 500.degree. C. there is an equal temperature drop/stage profile as compared to an equal chilling rate/stage profile in the staged chilling zone. The improvement is evidenced by increased feed filter rate.
2. Description of the Prior Art
It is well known that wax-containing petroleum oil stocks can be dewaxed by shock chilling with a cold solvent. It is also known that shock chilling, in itself, results in a low filtration rate of the dewaxed oil from the resultant wax/oil-solvent slurry. It is now well known that the harmful effects of shock chilling can be overcome by introducing the waxy oil into a staged chilling zone and passing the waxy oil from stage to stage of the zone, while at the same time injecting cold dewaxing solvent into a plurality of the stages and wherein a high degree of agitation is maintained in the stages so as to effect substantially instantaneous mixing of the waxy oil and solvent. As the waxy oil passes from stage to stage of the cooling zone it is cooled to a temperature sufficiently low to precipitate wax therefrom without incurring the harmful effects of shock chilling. This technique produces a wax/oil-solvent slurry wherein the wax particles have a unique crystal structure which provides superior filtering characteristics such as high filter rates and high dewaxed oil yields. The basic concept of dilution chilling dewaxing is disclosed in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference and will hereinafter be referred to as DILCHILL* for the sake of brevity. (*DILCHILL is a registered Service Mark of Exxon Research and Engineering Company).
A number of improvements and modifications have been made to the basic concept of DILCHILL. U.S. Pat. No. 3,642,609 shows that in a vertically staged cooling tower, the velocity of the solvent at the injection points within each stage should be at least 5-30 times that of the peripheral velocity of the mixer blades. This results in greater filtration rates and higher dewaxed oil yields than could otherwise be obtained without the relatively high velocity solvent injection. In U.S. Pat. No. 3,775,288 is disclosed a combination of dilution chilling with scraped surface chilling for dewaxing lubricating oils. U.S. Pat. No. 3,681,230 discloses adjusting the dewaxing solvent composition so that the waxy oil and solvent are immiscible near the last stage of the cooling zone. This results in superior dewaxed oil yields and higher filter rates when the waxy oil stock being fed to the tower is relatively high in viscosity and molecular weight. U.S. Pat. No. 3,850,740 discloses partially prediluting the waxy oil when same is relatively heavy feed such as a resid or a bright stock, before the oil is introduced into the chilling zone.
However, in all of these DILCHILL dewaxing processes it was thought that the rate of solvent addition to each stage should be adjusted so as to obtain the same or approximately equal temperature drops in each stage. For example, U.S. Pat. No. 3,773,650 in column 6, lines 7-11, discloses adding cold solvent so as to give equivalent temperature drops per stage. Similarly, in U.S. Pat. No. 3,775,288 at column 4, lines 38-43, it is disclosed that the same temperature drop should be maintained from stage to stage of the chilling zone.
U.S. Pat. No. 4,146,461 shows that the DILCHILL process is improved when the waxy lube oil stocks are solvent dewaxed by contacting them with successive increments of cold dewaxing solvent at a plurality of points along the height of a vertical tower divided into a plurality of stages while agitating the oil solvent mixture in each stage to provide substantially instantaneous mixing of the waxy oil and solvent thereby precipitating wax from the oil while avoiding the well known shock chilling effect by adjusting the cold solvent addition to each stage in a manner so as to modify the temperature profile along the tower to ensure that the temperature drop per stage in the initial stages in which wax precipitation occurs is greater than the temperature drop per stage in the final or later stages in which wax precipitation occurs.